1. Technical Field
The present disclosure relates to demodulation techniques of digital modulated signals in communication systems. Particularly and not exclusively the present disclosure refers to demodulation techniques applicable to the field of global navigation satellite systems.
2. Description of the Related Art
Satellite-based positioning systems include constellations of earth orbiting satellites that constantly transmit orbit data and ranging signals to receivers. An example of a satellite-based positioning system is the Global Positioning System (GPS). A GPS receiver receives the satellite signal information from at least four satellites and calculates the receiver location by measuring the range of the receiver from each used satellite and determining the accurate position of each used satellite in a suitable reference coordinate system. Accurate satellite position is computed from a specific set of data referred as ephemeris. For example the ephemeris data of a GPS satellite allows the receiver to compute the satellite position, in addition to its velocity, clock bias, and clock drift over a future time interval of approximately four hours. Therefore, a correct ephemeris data downloading is an important step that the conventional receiver has to perform to determine the position of the receiver. In the context of a GPS system the demodulation of the Binary Phase Shift Keying modulated data ephemeris is performed, according to a particular technique, through a Differential Binary Phase Shift Keying, D-BPSK, demodulation method. Moreover the time taken for a correct ephemeris data demodulation and decoding has a direct impact on the amount of time taken by the receiver to get the first fix, known as Time To First Fix, TTFF, once it is turned on.
However, there are many environmental situations that cause an attenuation of the received satellite's signal. Low signal strength conditions can occur in challenging environments such as urban canyons, under foliage, inside tunnels etc. The low signal strength condition can prevent the correct demodulation or decoding of the ephemeris data because of the consequential increase of the bit error rate, BER. So in low signal strength conditions a correct data ephemeris demodulation and decoding takes a longer time with respect, for example, to a theoretical 30 seconds.
U.S. Patent Application No. 2010/0134349, describes a system for data decode in a GPS receiver provided with a Data Inversion Prevention Algorithm subsystem, DIPA, having a Differential Binary Phase Shift Keying demodulator. The differential binary phase shift demodulator performs a difference between the I/Q phase value of the current bit and the I/Q phase value of the previous bit stored in a delay register, using a differential phase unit. Similarly, the I/Q phase value of the current bit is compared against the I/Q phase value corresponding to a bit that is two bits prior to the current bit. The I/Q phase value for this older bit is stored in a second delay register. The same phase difference computation method applies to a bit that is three bits and four bits prior to the current one. The above described four independent processing lines generate four independent bit decisions that are finally combined into a majority voting criteria unit that performs the final bit decision.